Compound (I) or a salt thereof to be used in the present invention has been reported as an inhibitor of dipeptidyl peptidase (DPP-IV) which is an enzyme that degrades glucagon-like peptide-1 (GLP-1), a hormone enhancing insulin secretion (WO2005/095381).
A report has been documented on the administration of compound (I) or a salt thereof to be used in the present invention in combination with metformin hydrochloride (WO2007/033266).
However, a preparation comprising compound (I) or a salt thereof, and metformin hydrochloride, which is superior in the preservation stability has not been reported.